1. Field of the Invention
The present invention relates in general to brakes for a wheeled motor vehicle, and more particularly to anti-lock braking systems for the vehicle, which automatically controls wheel slip or prevents sustained wheel-locking on braking. More specifically, the present invention is concerned with the anti-lock braking systems of a type which is suitable to a motor vehicle having at least three road wheels.
2. Description of the Prior Art
In the anti-lock braking systems of the above-mentioned type, when, due to hard braking or the like, the road wheels are about to be locked, the brake pressure applied to the brake cylinders of the wheels somewhat reduced and then somewhat increased in a manner not to cause the wheel-locking. That is, until the time when the speed ratio of the wheel speed to the vehicle speed comes to a critical value which may cause the wheel-locking, the brake pressure in the brake cylinders is kept constant, but when the speed ratio exceeds the critical value, the brake pressure is decreased, and when the pressure decreasing comes to a certain degree, the brake pressure is increased. With repetition of such decrease and increase in the brake pressure, the anti-lock braking is carried out with a permissible slippage of the wheels.
In order to compensate an unavoidable phenomenon wherein the wheel speed does not accurately correspond to the real vehicle speed while the wheel slip is taking place, a measure has been proposed by Japanese Patent First Provisional Publication 64-63452. In this measure, respective speed sensors are provided for the four road wheels, and pulse signals outputted from the sensors are processed to calculate the rotation speeds of the respective road wheels. Using the maximum one of the calculated four wheel speeds as a reference wheel speed, an estimated vehicle speed is derived. Based on the estimated vehicle speed, a slip rate of each road wheel and acceleration of the same, the anti-lock braking is carried out for effecting the compensation of the phenomenon.